Hydraulic stud tensioning and tightening devices are widely employed for applying a pre-determined axial load directly to a stud causing it to stretch and then be secured by a threaded nut in that tensioned condition. Conventional hydraulic stud tensioning and tightening devices are generally divided into two categories, both of which have disadvantages overcome by the present invention.
The first category of conventional stud tensioning and tightening devices comprises those wherein a piston and cylinder define a hydraulic pressure chamber which is annular and concentrically disposed around the stud. When the chamber is pressurized and the piston and cylinder move apart a load is applied to a so-called puller threaded onto the stud above its nut, thereby causing the stud to be stretched. A significant disadvantage of those designs is that the annular pressure chamber around the stud requires a measurable amount of free space and hence the outer diameter of the annular pressure chamber is often limited by proximity to adjacent studs on either side of the one being tensioned. Sometimes it is therefore necessary to tension only every other stud rather than all of them because of the space requirements of prior art devices. The inner diameter of the annular pressure chamber is of course limited by the necessary thickness of the wall of the cylinder which itself must surround the stud. Therefore the effective hydraulic working area of those stud tensioning and tightening devices which define an annular pressure chamber around the stud is limited and that is turn directly limits their loading capability.
The second category of prior art stud tensioning and tightening devices is those which depart entirely from conventional studs or nuts and resort instead to special designs. Examples are the devices described in U.S. Pat. Nos. 4,120,230 and 3,835,523 wherein the conventional stud is replaced by one with a special inner bore and the conventional nut is replaced by a unique element on the special stud. While the pressure chamber is circular and disposed beyond the end of the special stud and nut in both those designs, they apply the load to the stud by pushing on a compression column extending axially into a bore in the stud. It is self-evident that designs of tensioning and tightening devices which cannot be used with conventional studs and nuts are expensive and find little favor in nuclear, military and naval equipment where great reliance is placed upon standard and proven stud and nut design.
It is the principle purpose of the present invention to provide an improved stud tensioning and tightening apparatus which avoids the shortcomings of both categories of prior art designs described above. The apparatus of the invention avoids the use of an annular pressure chamber around the outside of the stud and instead places the pressure chamber coaxially beyond the outer end of the stud where it can be made circular rather than annular. This results in a much larger hydraulic working surface and increases the load-bearing capacity of a given form of the apparatus which might otherwise be limited by proximity to the studs and nuts on either side. It also makes possible the tensioning of all studs rather than every other one. The invention also constitutes an improvement over the second category of conventional designs in that it requires no change whatever in the standard configuration of studs and nuts. Not only can a given form of the invention be used to tension and tighten a standard stud and nut but with a simple change of parts a given form of the invention can be used for standard studs and nuts of different sizes.
There are other advantages of the invention as will become apparent from the following description.